1. Field of the Invention
This invention relates to a phase analog encoding system with compensation, used in connection with a resolver position transducer and utilized in servo control or monitoring applications.
A resolver position transducer is a device which monitors the position of a rotatable shaft or a linearly displaceable member by measuring the angular displacement of the shaft or the linear displacement of the member with respect to a fixed reference point. The resolver, when excited with the proper electrical input will output an electrical signal whose phase is related to the position of the shaft or member. Thus the position of the shaft or member is encoded in an electrical signal in an analog manner. By putting the electrical signal from the resolver position transducer through more encoding circuitry, an electrical signal can be obtained which represents the position of the shaft or member.
If one knows the position of the shaft or member, one can determine if a machine connected to the shaft or member is operating properly. Thus one is able to monitor the performance of a machine by measuring the angular displacement of a rotatable shaft or the linear displacement of a linearly displaceable member with a resolver position transducer.
Additionally, if one knows the position of a machine's shaft or member, one is able to use that information in a feedback network to control the operation of the machine in order to obtain any desired performance. Used in this manner, the resolver position transducer is performing a servo-control function.
2. Description of the Prior Art
The phase shift errors inherent in a resolver position transducer are of particular importance in applications where the resolver is part of a phase analog encoding system. The phase analog encoding technique utilized in such systems involves applying reference signals to the resolver position transducer in the form of two sinusoidal signals displaced in time by 90 electrical degrees such as: EQU VR1=K.sub.1 SIN.omega.t (1) EQU VR2=K.sub.1 COS.omega.t (2)
where VR1 is the voltage across the equivalent of a stator sine winding, VR2 is the voltage across the equivalent of a stator cosine winding and K.sub.1 is a constant. Feedback from the resolver is taken by measuring the voltage across the equivalent of a resolver rotor winding, VFB. If VR1 and VR2 are applied to the equivalent stator sine and cosine windings, the equivalent resolver rotor winding has a voltage of the form: EQU VFB=K.sub.2 SIN(.omega.t+.phi.+.alpha.) (3)
where .phi. is the mechanical displacement of a rotatable shaft or a linearly displaceable member, .alpha. is the inherent electrical phase shift across the windings of the resolver position transducer, and K.sub.2 is a constant. If the resolver position transducer is monitoring a rotatable shaft, the mechanical displacement .phi. is an angular displacement. If the resolver position transducer is monitoring a linearly displaceable member, the mechanical displacement .phi. is a linear displacement.
The typical phase analog encoder operates by measuring the relative phase difference (i.e., phase shift) between one of the reference signals (1) or (2) and the feedback signal (3). This measured phase shift is equal to the sum of the mechanical displacement .phi. and an offset value which is the electrical phase shift across the equivalent stator and rotor windings of the resolver .phi..
The above encoding technique for measuring the mechanical displacement .phi. will be accurate as long as .alpha. remains constant. Usually .alpha. does not vary by more than one or two degress. As a result, the overall phase analog encoding system utilizing this technique is low cost, easy to apply and very effective for applications where an accuracy of one or two degrees is acceptable.
For many applications, however, the change in .alpha. with respect to: temperature; variations in input frequency; manufacturing tolerances and other mechanical constraints such as shaft loading; can be quite large, requiring some form of compensation. Typically, the most severe errors are introduced by variations in temperature. As a result, some form of compensation is necessary where the resolver will be operating in an environment with wide variations in the ambient temperature. Compensation is also necessary where the ambient temperature is constant but the resolver is attached to a device such as a motor which varies in temperature depending on how long the device has been operating.
There are two common forms of compensation for variations in temperature. One form involves mounting a temperature sensor in a network to compensate for the inherent electrical phase shift. The second form involves the use of an additional winding in the resolver position transducer and a separate encoding circuit which is used to monitor the electrical phase shift across the additional winding so that a compensating signal can be generated which is then used to correct the primary encoding circuitry of the resolver position transducer.
The disadvantage of both of the common forms of temperature compensation is that they involve additional components and more complex circuitry. This increases the total cost of the system and increase the possibility of component failure and system break down.